In prehistoric times people were not yet busy with time. They were completely unaware of minutes and hours. They woke up when it got light and went to sleep as soon as the sun went down.

We now know that the body has a mechanism to sleep and wake up on time. This is your biological clock. The biological clock that controls the pattern of sleeping and waking is also called the circadian clock. The alternation of dark and light (day and night) is necessary for this clock to coincide with the environment.

Many animals have a biological clock that is linked to the seasons. A kind of internal alarm ensures that they eat certain quantities and start hibernation in time.